Rhodamine is a fluorescent dye, long known in the same fashion as fluorescein. Both have high fluorescence quantum yield in water and have therefore been widely applied in the field of biology as a fluorescent tag. Living-cell imaging that employs a fluorescent probe is frequently used in recent years, and rhodamine is also widely used as a mother nucleus of a fluorescent probe that plays a major role in living-cell imaging techniques.
Reported examples of fluorescent probes having a rhodamine skeleton include those used for detecting nitrogen monoxide (Patent Reference 1) and hypochlorous acid (Patent Reference 2). Also, already reported are a compound (TMDHS: 2,7-N,N,N′,N′-tetramethyl-9-dimethyl-10-hydro-9-silaanthracene) resulting from substituting with a silicon atom the oxygen atom of pyronin, which is the basic skeleton of rhodamine, and application of the compound to a fluorescent probe (Non-patent References 1 and 2).
In fluorescent probes having TMDHS as a basic skeleton, intramolecular photo-induced electron transfer (PET) and the open or closed ring of a spiro ring are essentially used to turn fluorescence on or off. However, TMDHS or other compounds resulting from substituting with a silicon atom the oxygen atom of pyronin as reported in the past have the amino group at positions 2 and 7 substituted with a methyl group or other substituent group other than a hydrogen atom. There have yet to be any reports of a rhodamine analogue resulting from substituting with a silicon atom the oxygen atom of a compound in which the substituent group of amino groups at position 3 and position 6 has an asymmetrical structure in rhodamine, nor have there been any reports of a fluorescent probe that uses such a rhodamine analogue.
Several fluorescent probes for detecting active oxygen that are used in near-infrared fluorescent imaging have been developed, but these present problems in regard to the small increase in fluorescence after active oxygen has been detected and poor selectivity in relation to the active oxygen species.